FM transmitters can impress voice signals on a radio frequency by applying a voiceband voltage signal to a voltage controlled oscillator operating at an output frequency. The mean frequency is accurately controlled to a desired channel frequency relative to an accurate crystal reference with the aid of a phase-lock loop or frequency synthesizer. The voiceband signal causes instantaneous deviation of the frequency from the mean in step with the voice waveform from the microphone.
This relationship between voice signal and frequency deviation generally should be controlled to tight tolerances. Too great a deviation may cause the signal to stray momentarily into neighboring channels causing "splatter" interference. Too little deviation may reduce the speech volume at the receiver and impair intelligibility under noisy conditions. The uncertainty in deviation in such FM transmitters arises from the variability of the sensitivity of the voltage controlled oscillator to the voiceband signal, expressed in Megahertz of frequency deviation per volt.
At least two known prior art radio products employed digital scaling of frequency modulating signals to compensate for voltage controlled oscillator sensitivity variation. A radio offered by the Marconi company between about 1980-1985 known as SCIMITAR-V employed a Read Only Memory to store modulation scaling coefficients as a function of Voltage Controlled Oscillator (VCO) frequency or channel number. A derivation of this radio known as STARCOMM was jointly developed by Marconi of England and Ericsson (the current assignee) and included the same technique for compensating VCO slope variation.
This technique is generally described below with reference to FIG. 1. As shown in FIG. 1, controlled oscillator such as a VCO 10 produces an output at point B that is frequency modulated by a modulating signal presented to the input at point A. The VCO is controlled to oscillate at a desired channel center frequency by the fractional-N synthesizer loop comprising loop filter 11, phase error detector 12 and fractional-N synthesizer or divider 13. More details of the operation of fractional-N synthesizers, phase detectors and design of loop filters can be found in U.S. Pat. Nos. 5,095,288 and 5,180,933 to the present inventor, which patents are hereby incorporated by reference herein.
The modulation signal from input A frequency modulates the output signal frequency via two paths, known as two-point modulation. The two paths are sometimes known as "in-loop" and "out-of-loop" modulation, or "closed loop" and "open loop" modulation. The closed loop modulation is applied by digitizing the modulation signal using analog to digital convertor 14 to obtain a series of numerical samples representing the modulating waveform. The numerical sample values are digitally added to the channel code N+dN by the fractional-N synthesizer 13 to determine the instantaneous channel frequency plus the instantaneous frequency deviation due to modulation. The fractional-N divider 13 then causes the phase lock loop to attempt to control the VCO 10 to this value.
When the modulation attempts to change frequency more rapidly than the loop can follow, the modulation may not faithfully be transferred to the VCO due to the bandwidth limitation imposed by loop filter 11. The open loop modulation may then be applied to the VCO directly, bypassing the loop filter. This can be thought of as forewarning the VCO of the coming frequency change the loop will demand. The VCO is then able to adopt the new frequency more or less simultaneously with the phase lock loop demanding the change, so that reduced error occurs in the loop. Therefore, the loop filter may not impose a bandwidth limit on the modulation.
The direct modulation is preferably applied to the VCO at a point where a flat modulation frequency response will result. It can, in principle, be applied by voltage addition after loop filter 11, but it is undesirable to attach additional components to the VCO control line due to its extreme sensitivity to interference pick up. Therefore the direct modulation is preferably applied as another input to the loop filter, using components .alpha..multidot.Ro and Co/.alpha. as impedance-scaled versions of the loop filter components Ro,Co. Since the time constant of (.alpha..multidot.Ro)(Co/.alpha.) is the same as RoCo, the resulting frequency response is flat and scaled down by the factor .alpha. from the modulation sensitivity available on the VCO control line. The series capacitor Co/.alpha. also has the desirable effect of isolating the VCO from whatever DC level may exist at point A, which may not be the same as the DC level on the VCO control line since this is frequency channel dependent.
When the VCO has to be tuned over a wide range of channel frequencies, the frequency versus control voltage is unlikely to be perfectly linear, so that the tangential modulation sensitivity dF/dV generally is not of constant slope but varies with frequency. This characteristic curve was nevertheless anticipated to be constant in the design of FIG. 1 and scaling values compensating for this fixed curve are precomputed and stored in ROM 16. Indeed, the VCO adjustment needed in any case in factory production can be used to ensure that the nominal curve of tangential modulation sensitivity versus frequency channel is reproduced in each unit. Compensation for the variation of tangential modulation sensitivity versus frequency can be achieved by scaling the direct modulation signal using a multiplying DtoA convertor 15 which multiplies an analog input signal at point A by an 8-bit digital number representing values between 0/256 and 255/256 in steps of 1/256.
The 8-bit scaling constants to equalize the modulation deviation at all channel frequencies are precomputed and stored in memory such as read only memory (ROM) 16. These values were generally not adapted from one model to another and were generally not updated after delivery from the factory. In order to use a fixed ROM table of scaling values, however, manual alignment and adjustment of the VCOs in the factory may be required in order to achieve an expected tangential modulation sensitivity curve.